Typical idling speed for a well-warmed late '70s V8 engine?
 

Anyone want to offer a reply?

My Britannia approx 560RPM in neutral 500 in drive idling

Never measured it accurately, but my 603 will tick-over below 400. Doesn't sound nice so low so a rev-counter needle's width above 500 is normal. That's with the AFB carb - with the old Thermoquad, it wouldn't tick-over cleanly below about 800 - but that leaked fuel and air in equal quantities!

David